The invention relates to a system for the transmission of data packets of mutually equal durations from a plurality of substations to a main station via one or more transmission channels, the main station comprising transmission means for transmitting on each transmission channel a synchronising signal which indicates the beginning of a frame on reception of the synchronising signal on a transmission channel, each substation subdivides the frame into a number of consecutive time slots of mutually equal durations, and transmits, if so desired, a data packet in a time slot which is selected from the sequence of time slots.
The above system is commonly known as a "slotted ALOHA" system, a particular version of which is described in the UK publication "MPT 1327--A Signalling Standard for Trunked Private Land Mobile Radio Systems", Mar. 26, 1987. This standard requires that the substations, in this instance radio units, shall commence RF transmission, reach 90% of maximum power, transmit the data packets and cease RF transmission so that power is reduced by 60 dB, within specified periods in the time slots. Furthermore in the situation where there exists a forward channel (main station to substations) and a different frequency return channel (substations to main station) the standard requires that the substation shall then return to the forward channel in time to be capable of decoding address codewords. If any one substation does not meet these requirements of the standard it is not permitted to transmit. Otherwise the transmission by a substation will result in crosstalk and corruption of data of that transmission and transmissions of other substations.
This system has the disadvantage that these timing constraints require substations having very fast power up, power down and retuning times which are difficult to implement and result in costly substations. In addition, these timing constraints do not allow for a mixture of substations having different timing parameters. Moreover these timing constraints require an infrastructure, e.g., repeaters, base stations, satellites, etc., also to have fast power up and down times. Any repeaters for instance that have slow power up and down times will result in corruption of the data packets. Thus the implementation of the known system in existing networks may require the upgrading of the infrastructure.